<P dir=ltr align=left><STRONG>Introduction to Earthquake Geology</STRONG></P>
Earthquake geologists study the surface expression of earthquake faults and ground failure caused by strong ground shaking. An important facet of earthquake geology is paleoseismology, the study of the timing, location, and size of prehistoric earthquakes. Paleoseismologists typically excavate trenches across known active faults to expose and examine the evidence left by large, surface-rupturing earthquakes. On any given fault, the interval between large damaging earthquakes is typically hundreds to thousands of years, usually much longer than the historical record of earthquakes. Knowledge of when, where, how often, and with what magnitude large earthquakes occur is crucial for understanding and characterizing the seismic hazard of a region. The amount that geologic features are offset across a fault can provide an important gauge of the rate of fault movement, and thus of earthquake activity, as averaged over multiple earthquake cycles. Ground deformation phenomena, such as tectonic subsidence and uplift, liquefaction, and shaking-induced landslides, can be used to identify earthquakes in the geologic record and to evaluate the scope of the earthquake hazard faced by society.